1. Field of the Invention
The present invention relates to a projector unit for projecting an image based on image information such as a video signal and more particularly to a projection side optical system of a projector.
2. Description of the Related Art
Currently, in many cases, video projectors are used as image projection units for projecting, for example, a screen of a personal computer and a video image on to a screen and the like.
In projectors of this kind, a high intensity light source is incorporated, and light from the light source is shone through a light source side optical system where the light is sequentially split into red, green and blue light by the use of a red filter, a green filter and a blue filter. Then, these beams of red light, green light and blue light are concentrated into a light modulator such as a micromirror display device which is referred to as a liquid crystal device or a DMD (Digital Micromirror Device) by means of lenses and the like which make up an illumination side optical system, so that the light beams are then transmitted or reflected to be directed towards a projection opening in the projector while being controlled with respect to their quantities so as to display a fullcolor image on the screen.
The micromirror display device is such that microscopic mirror cells are tilted separately by control signals so as to control the direction of reflected light between an ON position, where light made to enter the micromirror display device by the illumination side optical system is reflected towards the projection lens which makes up the projection side optical system, the light so reflected being herein referred to an ON-state light, and an OFF position where the light that has so entered the micromirror display device is reflected towards a light absorber, the light so reflected being herein referred to as an OFF-state light, whereby time during which the red light, green light and blue light are reflected by the micromirror cells which are in the ON position is controlled so as to project a full-color image on to the screen.
In addition, there has been proposed a technology in which the white balance of a projector is regulated by making use of light which is reflected by micromirrors in the OFF position, more specifically, by detecting light which is reflected by the micromirrors in the OFF position so as not to be incident on the projection lens (for example, refer to the Japanese Unexamined Patent Publication No. 2001-188).
In addition, the liquid crystal display device is such as to cut off or transmit light which passes through microscopic liquid crystal cells so as to form an image, and in this liquid crystal display device, when focusing the light that has passed through the liquid crystal cells on to a screen by a projection lens, it has been proposed that a triangular opening is formed in the projection lens or an aperture is provided therein in order to enhance the contrast of an image formed (for example, refer to the Japanese Unexamined Patent Publication No. 2004-157346).
However, when increasing the opening in the illumination side optical system or the projection side optical system with a view to enhancing the brightness of an image projected by the projector, due to the expansion of the opening in the illumination side optical system, the diameters of a beam of ON-state light which is a beam of light made up of the ON-state light, a beam of OFF-state light which is a beam of light made up of the OFF-state light and a beam of stray light which is a beam of flat stray light which is a reflected light from a flat portion lying on the periphery of a cover glass of the display device or the mirror cells are also expanded.
Because of this, a chance of such stray light entering the projection lens which makes up the projection side optical system is increased and the amount of stray light which enters the projection lens is also increased in association with the expansion in diameter of the opening in the projection side optical system, resulting in a drawback that the contrast of an image formed is deteriorated by the stray light and the like.
The invention was made in view of the situations and an object thereof is to provide a projector lens which can form a bright and sharp image on to the screen by eliminating the drawback inherent in the related art and, hence, a projector which can obtain a sharp image.